Various means have been used in the past to determine whether the gap between the shell body and the base plate of a projectile is excessive. It has been determined that under certain firing conditions assembly pins holding parts together are sheared and the base plate driven forward against the shell body. This movement, in turn, drives the fuze mechanisms and charges in the shell body forward, and on occasion has caused premature, in-bore detonation of the shell, with resultant catastrophic effect on the launch weapon and the gun firing crew.
In order to accurately measure the gap between the shell body and the base plate, the prior art utilized inspection techniques consisting of manually checking the periphery of the body-base joint with a feeler type hand held gauge. This method has been found to be unacceptable due to an inability to measure all types of out of tolerance conditions. This difficulty in determining out of tolerance gap distances is particularly prevalent in the instance where the rim of the base plate is thicker than the inner portion. In such circumstances a "no-go" feeler gauge used to measure the gap would indicate a "good" assembly, while in fact the assembly would be out of tolerance and probably hazardous to fire. In addition, the feeler gauge method has been found to be unreliable as a means for accurately measuring a gap in a projectile assembly because it was always dependent upon a subjective human response which could vary from person to person and from one time to another.